The present invention relates to the removal of heat stable amine salts from liquid streams. More specifically, the present invention relates to processes for 1, converting an amine in salt form to an amine in free base form and 2, removing the anion of the heat stable amine salt as a salt of the associated base that has been added to a modified electrodialysis zone.
A wide variety of absorption processes have been proposed for removing acid gases such as, for example, carbon dioxide, hydrogen sulphide, sulphur dioxide, sulphur trioxide, carbon disulphide, hydrogen cyanide, and carbonyl sulphide from process gas streams using absorbents comprising amines.
Such absorption processes typically involve passing the process gas stream containing one or more of the acid gases to an absorption zone wherein it is contacted with a lean solvent comprising the amine absorbent. A product gas stream, depleted in the acid gases relative to the process gas stream, is withdrawn from the absorption zone as a product. A rich solvent stream comprising the amine absorbent and the absorbed acid gases is also withdrawn from the absorption zone and passed to a regeneration zone, e.g. a stripping column, wherein the absorbed acid gases are desorbed from the solvent to provide a tail gas stream comprising the acid gases and the lean solvent stream herein before described.
A common problem in such acid gas absorption processes is that heat stable salts of the amine are often formed during one or both of the absorption and regeneration steps as a by-product. Heat stable salts can be formed, for example, when strong acids such as hydrochloric acid or sulphuric acid are present in the process gas.
Heat stable salts can also be formed when sulphite anions are oxidised to sulphate anions in SO2 amine recovery processes. Typical ions which form heat stable salts, i.e., heat stable anions, include, for example, sulphate anions, thiosulphate anions, polythionate anions, thiocyanate anions, acetate anions, formate anions, nitrate anions, chloride anions, oxylate ions and in addition for amines suitable for H2S and CO2 scrubbing, sulphite anions. Heat stable salts generally do not have absorption capacity for the acid gases and are not regenerable under the conditions of the process. Therefore, the level of heat stable salts needs to be controlled in order to retain an adequate degree of absorption capacity for the acid gases.
Electrodialysis has been proposed as a method for removing heat stable salts from amine containing streams. In a typical electrodialysis process, (ref U.S. Pat. No. 5,910,611) caustic, e.g., sodium hydroxide, is added to the stream containing the heat stable amine salt in order to dissociate the heat stable anion from the heat stable salt and provide an amine in free base form and a simple heat stable salt, e.g., sodium sulfate. The simple salt is then separated by conventional electrodialysis wherein the charged ions permeate through anion- and cation-selective membranes. The amine, which is non-ionic, does not permeate through the membranes and is discharged from the electrodialysis zone as a product. Often, conventional electrodialysis processes operate in a batch mode wherein the process streams are recirculated until the desired amount of heat stable salts is removed.
Certain problems can result from the use of electrodialysis processes such as described above. For example, since the amine product from the electrodialysis zone is provided in free base form, it can have excessive volatility which can lead to solvent losses during absorption. In addition, when the process is a batch process, the pH and ionic strength within the compartments of the electrolysis zone vary with the discontinuous operation. As a result, the membranes in the electrodialysis zone often experience shrinking and swelling and, ultimately, are subject to mechanical failure. Moreover, to the extent that the amine is not converted to free base form in the caustic treatment step, there can be substantial losses of the amine due to permeation through the membranes in the electrodialysis zone. In addition, there can be significant losses through the membranes as a result of osmostic forces in the conventional electrodialysis process.
An alternative electrodialysis process (U.S. Pat. No. 5,292,407) achieves the removal of the heat stable anion by substituting the heat stable anion with a regenerable non-heat stable anion in a modified electrodialysis zone. In this process, the regenerable anion is introduced into the electrodialysis stack as an acid, such as sulfurous acid, in the case of an SO2 scrubbing process. Neutralizing the heat stable amine salt with a base in order to generate an amine in free base form is not required in this case. Amine losses are significantly reduced. This process suffers from the disadvantage that the regenerable anion must be introduced as an acid with the resultant low pH and associated corrosion or as a salt such as sodium bisulphite, or sodium carbonate. These salts are typically sold as solids implying that they must be mixed and dissolved in addition to often containing significant quantity of impurities. If the regenerable anion is introduced as an acid, then the materials of construction must be chosen such that they are resistant to the low pH environment. In acid form, the regenerable anions often possess significant vapor pressure, resulting in operational difficulties and process hazards in the case of SO2 or H2S. The waste stream will also be composed of acidic species of the non-regenerable acid species, and in most instance would require neutralization before being disposed of.
In accordance with the present invention, processes are provided which utilize a modified electrodialysis zone in order to convert heat stable amine salts in an amine solution to salts of a base other than the amine in which they were originally associated with and amine in free base form. The process of the present invention can provide a high degree of recovery of the amine in the electrodialysis zone, requires the addition of a base such as caustic and can be highly integrated with acid gas-absorption process. In an alternative embodiment, the addition of caustic can be eliminated by the utilization of bi-polar membranes (water splitting) in which OHxe2x88x92 and H+ ions are generated by dissociation of water molecules.
More specifically, the present invention provides a process for converting heat stable amine salts to free base amine and non-amine salts which comprises:
(a) passing a feedstream comprising an amine in salt form having heat stable anions associated therewith to an electrodialysis zone having a cathode compartment, an anode compartment and at least one repeat unit comprising a base or anion source compartment disposed between the cathode compartment and the anode compartment, a product compartment disposed between the base feed compartment and the anode compartment, a feed compartment disposed between the product compartment and the anode compartment, and a salt compartment disposed between the feed compartment and the anode compartment, said regenerated solvent stream being passed to the feed compartment;
(b) passing a base stream comprising a base such as caustic or equivalent base to the base compartment;
(c) passing a direct current potential transversely across each compartment, said current being effective to cause: (1) amine cations to dissociate from the heat stable amine salts in the feed compartment and pass into the product compartment and be neutralized into its free base form; (2) hydroxide or equivalent anions to be dissociated in the base compartment and pass into the product compartment; and (3) heat stable anions to dissociate from the heat stable amine salts in the feed compartment and pass into the salt compartment; and (4) base cations to dissociate in the base compartment and pass into the salt compartment; and
(d) discharging from the product compartment a product stream comprising an amine in salt form having at least a portion of the amine in free base form.
A second embodiment of the same invention which also provides a process for converting heat stable amine salts to free base amine or regenerable amine salts and non-amine salts which comprises:
(a) passing a feedstream comprising an amine in salt form having heat stable anions associated therewith to an electrodialysis zone having a cathode compartment, an anode compartment and at least one repeat unit comprising a anion source compartment disposed between the cathode compartment and the anode compartment, a combined feed and product compartment disposed between the base feed compartment and the anode compartment, and a waste compartment disposed between the combined feed-product compartment and the anode compartment, said regenerated solvent stream being passed to the feed-product compartment;
(b) passing a base stream, or an acid or salt with a regenerable anion, such as caustic in the case of a base or sodium carbonate in the case of a salt with a regenerable anion to the anion source compartment;
(c) passing a direct current potential transversely across each compartment, said current being effective to cause: (1) amine cations to dissociate from the heat stable amine salts in the combined feed and product compartment and be neutralized to its free base form; or into a regenerable form (2) hydroxide or equivalent anions to be dissociated in the anion source compartment and pass into the combine feed and product compartment; and (3)heat stable anions to dissociate from the heat stable amine salts in the feed-product compartment and pass into the waste compartment; and (4) base, acid or salt cations to dissociate in the anion source compartment and pass into the waste compartment; and
(d) discharging from the feed-product compartment a product stream comprising an amine in salt form having at least a portion of the amine in free base form or in a regenerable form. A third embodiment of the same invention which also provides a process for converting heat stable amine salts to free base amine and the corresponding acid of the heat stable anions comprises:
(a) passing a feedstream comprising an amine in salt form having heat stable anions associated therewith to an electrodialysis zone having a cathode compartment, an anode compartment and at least one repeat unit comprising a waste (concentrate) feed compartment disposed between the cathode compartment and the anode compartment, a combined feed and product compartment disposed between the waste compartment and the anode compartment, said regenerated solvent stream being passed to the feed-product compartment;
(b) passing a direct current potential transversely across each compartment, said current being effective to cause: (1) amine cations to dissociate from the heat stable amine salts in the combined feed and product compartment and be neutralized to its free base form by hydroxide anions; (2) hydroxide or equivalent anions to be produced in the bi-polar membranes and pass into the combine feed and product compartment; and (3) heat stable anions to dissociate from the heat stable amine salts in the feed-product compartment and pass into the waste compartment; and (4) protons to be generated in the bi-polar membranes and pass into the waste compartment; and
(c) discharging from the feed-product compartment a product stream comprising an amine in salt form having at least a portion of the amine in free base form.